Alpha-methyl-styrene (aMS) is a technologically interesting monomer for homo- or copolymerization because of the resulting high glass transition temperature (Tg) of the polymers incorporating it. Inclusion of aMS as a comonomer with monovinyl-aromatic monomers such as styrene allows tailoring of the Tg of styrenic blocks. This is particularly suitable in anionic polymerizations where both aMS and styrene readily polymerize. Further, such high temperature glassy blocks can be expected to increase the service temperatures of thermoplastic block copolymers into which they are incorporated.
One well known problem associated with aMS polymers is their low ceiling temperature (Tc). The ceiling temperature is the temperature above which a polymer can spontaneously depolymerize. This presents a problem during polymerization where temperatures must be carefully maintained below Tc. Further, in elevated temperature applications aMS homopolymers will thermally decompose readily above Tc once decomposition is initiated by any process. One approach used to mitigate this problem is to copolymerize aMS with comonomers having high Tc. Such is the case with copolymers of aMS and styrene.
As second problem, heretofore unsolved, is the polymerization of aMS polymers, particularly copolymers with monovinyl-aromatic monomers, in which high conversions of aMS are achieved. This is important for the practical reason of avoiding difficult and expensive recycle operations. Also, in block copolymerization efficient consumption of the aMS monomer can prevent contamination of non-glassy blocks with the high Tg aMS.
Block copolymers have previously been prepared comprising aMS/S endblocks as taught in GB Patent 1,264,741. The polymerization was conducted to synthesize blocks having randomly distributed comonomers. The glassy endblocks of these block copolymers comprised a majority of aMS and between 10 and 40 mol % of styrene monomer. Importantly, the process required to synthesize these block copolymers was conducted at cold or cryogenic temperatures from −10 to −100° C.
Tapered block copolymers containing aMS have been taught in U.S. Pat. No. 4,427,837. The process taught therein suffered from conversions of less than 10% of the aMS monomer. Thus, large excesses of aMS monomer were required to incorporate modest amounts in the final polymer. Recovery of the unpolymerzied aMS monomer presents a significant practical problem.
Fully hydrogenated styrene/aMS/diene block copolymers have been taught in US Patent Application 2003/0065099. Small amounts of polar co-solvent were utilized in order to facilitate cross-over from the butadiene to the styrene/aMS. However, this process also suffered from very low conversions of the aMS monomer.
There yet exists a need for a process to copolymerize aMS/monovinyl-aromatic monomers which can be conducted at practical polymerization temperatures and which achieves high conversion of the aMS monomer. Such a process will allow the construction of block copolymers having high Tg glassy blocks and such block copolymers will be useful in making high service temperature rubber compounds. The present invention is directed to such a process, high Tg block copolymers and rubber compounds made from them.